An ever increasing number of people are being afflicted with cancer. Conventional treatments such as surgery, radiation therapy and chemotherapy have been extremely successful in certain cases; in other instances, much less so. A much less familiar, alternative form of cancer therapy known as Boron Neutron-Capture Therapy (BNCT) has been used to treat certain tumors for which the general methods are ineffective. In particular, it has been used to treat Glioblastoma multiformae, a highly malignant, invasive form of brain cancer. In this therapy, a patient is injected with a boron compound highly enriched in B.sup.10. The boronated compound concentrates preferentially in the brain tumor, while the action of the blood-brain barrier prevents its entry into the healthy surrounding tissues. The patient's head is then irradiated with a beam of thermal neutrons that are captured by the boron concentrated in the tumor according to the B.sup.10 (n,.alpha.)Li.sup.7 reaction. The tumor is thus irradiated with high LET alpha and Li particles whose range in tissue is about 10.mu., or the diameter of an average cell. Therefore, a very localized, specific reaction takes place whereby the tumor receives a large radiation dose, compared to that received by the surrounding healthy tissue, from the transit of the thermal neutrons.
Clinical trials of BNCT were conducted in the 1950's at Brookhaven by Farr; Farr, L. E., Applications of Radioisotopes & Radiation in the Life Sciences, U.S. Govn't Printing Office, March 1961; Sweet, Robertson and Stickley et al, Amer. J. of Roentgenology, 73, 279-293, 1954; and in the 1960's by Brownell, Sweet, and Soloway working at MIT; Asbury, A. K., et al, J. of Neuropath & Expt. Neurol., 31, 278-303, 1972; but with limited success.
Subsequent studies revealed two major problems. First of all, the boron compound was present in the skin and the blood vessels at concentrations equivalent to or greater than that in the tumor itself, resulting in severe damage to the blood vessel walls and the scalp directly in line with the treatment port. However, the problem appears to have been solved with the development of a low toxicity boron compound, Na.sub.2 B.sub.12 H.sub.11 SH. Tumor-to-blood boron concentration ratios of as high as 3:1 have been attained 12-24 hours after injection, Hatanaka, H., J. of Neurol., 207, 81-94, 1975.
The primary physical problem encountered was the rapid attenuation of the thermal neutron flux, preventing effective treatment of deep-seated tumors. A large proportion of the thermal neutrons never reach the tumor, but instead damage normal tissue. As a possible solution, substitution of the thermal beam by an epithermal beam has been proposed.
The rational for epithermal (intermediate) neutrons is that these neutrons pass through the outer layers of tissue losing energy in the process. After sufficient attenuation by tissues, the epithermal neutrons are slowed to the thermal energy range where they are subject to a high probability of capture by the B.sup.10. In order that an epithermal beam may be used for neutron-capture tumor therapy, a relatively safe epithermal beam must be provided. That is, an epithermal beam must be provided which has sufficiently low fluxes of destructive fast neutrons, thermal neutrons and gamma rays. Several attempts have been made at constructing an epithermal beam by scattering from a thin hydrogeneous material or by filtering a thermalized beam with Cd or Li.sup.6, Frigerio, N. A., Phys. in Med. & Biol., 6, 541-549, 1962, Fairchild, R. G., Phys. in Med. & Biol. 10, 491-504, 1965. However, either the gamma and/or fast neutron contamination was too high for therapeutic purposes, or the epithermal flux was too low. The need continues for a satisfactory means for providing an epithermal beam suitable for BNCT.